TROY Diamond,
CustomMessages вам в помощь:
Код:

[Setup]
ShowLanguageDialog = yes
AppId=MySuperProgramm
AppName={cm:AppName}
AppPublisher={cm:AppPub}
AppPublisherURL={cm:AppURL}
AppCopyright={cm:AppCopyright}
AppVersion=1.0.0.0
DefaultDirName=111
[Languages]
Name: ru; MessagesFile: compiler:Languages\Russian.isl
Name: en; MessagesFile: compiler:Default.isl
[_Code]
{...
MsgBox(CustomMessage ('MsgBoxText'), mbInformation, MB_OK);
MsgBox(ExpandConstant ('<<<< {cm:MsgBoxText} >>>>>'), mbInformation, MB_OK);
...}
[CustomMessages]
en.AppName=Program
en.AppPub=Publisher
en.AppURL=http://site.domain/en/
en.AppCopyright=Copyright
en.MsgBoxText=It's alive!
ru.AppName=Программа
ru.AppPub=Издатель
ru.AppURL=http://site.domain/ru/
ru.AppCopyright=Скопировано направо
ru.MsgBoxText=Таки работает!!!